The invention relates to a picture display device having a vacuum envelope which is provided with a transparent face plate with a display screen having a pattern of luminescent picture elements (pixels), and a rear plate, said display device comprising a plurality of juxtaposed electron propagation means extending substantially parallel to the face plate, between said propagation means and the face plate, an addressing system for addressing desired pixels and an apertured spacer plate of electrically insulating material for passing electrons.
The display device described above is of the flat-panel type, as disclosed in EP-A-464 937. Display devices of the flat-panel type are devices having a transparent face plate and, arranged at a small distance therefrom, a rear plate, while the inner surface of a face plate is provided with a (for example, hexagonal) pattern of phosphor dots. If (video information-controlled) electrons impinge upon the luminescent screen, a visual image is formed which is visible via the front side of the face plate. The face plate may be flat or, if desired, curved (for example, spherical or cylindrical).
The display device described in EP-A-464 937 comprises a plurality of juxtaposed sources for emitting electrons, local electron propagation means cooperating with the sources and each having walls of high-ohmic, electrically substantially insulating material having a secondary emission coefficient suitable for propagating emitted electrons, and an addressing system comprising electrodes (selection electrodes) which can be driven in rows so as to withdraw electrons from the propagation means at predetermined extraction locations facing the luminescent screen, further means being provided for directing extracted electrons towards pixels of the luminescent screen for producing a picture composed of pixels.
The operation of the picture display device disclosed in EP-A-464 937 is based on the recognition that electron propagation is possible when electrons impinge on a wall of a high-ohmic, electrically substantially insulating material (for example, glass or synthetic material), if an electric field of sufficient power is generated over a given length of the wall (by applying a potential difference across the ends of the wall). The impinging electrons generate secondary electrons by wall interaction, which electrons are attracted to a further wall section and in their turn generate secondary electrons again by wall interaction, and so forth.
Starting from the above-mentioned principle, a flat-panel picture display device can be realised by providing each one of a plurality of juxtaposed "compartments", which constitute propagation ducts, with a column of extraction apertures at a side which is to face a display screen. It will then be practical to arrange the extraction apertures along "horizontal" lines extending transversely to the ducts. By adding selection electrodes arranged in rows to the arrangement of apertures, an addressing means is provided with which electrons can be selectively withdrawn from the "compartments", which electrons can be directed (and accelerated) towards the screen for producing a picture composed of pixels by activating the pixels.
EP-A-464 937 particularly describes a multi-stage addressing or: selection system. A multi-stage selection system using a number of preselection extraction locations, which number is reduced with respect to the number of pixels, and directly or indirectly associated therewith a number of (fine-)selection apertures which corresponds to the number of luminescent pixels provides advantages with respect to, for example, the extraction efficiency and/or the required number of electrical connections/drivers. For controlling the preselection locations a pattern of preselecting electrodes is used, and for controlling the (fine) selection apertures a pattern of fine selection electrodes is used.
The luminescent screen is also referred to as the flu screen. An important component of the relevant display device is the flu spacer.
The flu spacer is located between the fine-selection electrodes and the face plate of the flu screen. Due to the efficiency and the saturation behaviour of the flu screen it is of paramount importance that the voltage between the flu screen and the fine selection electrodes is as high as possible. Dependent on the phosphors used, 3 kV or, even more frequently, 5 kV is a minimum requirement.
The fine-selection plate, the flu spacer and the flu screen are made of an insulating material, for example glass. A metallization of, for example nickel is provided in patterns on the fine-selection plate. A low-ohmic transparent conducting layer of, for example ITO is provided on the flu screen. This layer is provided with the flu screen and (possibly) a black matrix; the flu current is depleted via the conducting layer. A typical thickness of the flu spacer is 0.5-1.0 mm. The voltage difference between the fine-selection electrodes and the ITO layer on the flu screen should be as high as possible. A number of unwanted effects may occur at large voltage differences.